Ctrl+Shift+Y is a shortcut that opens downloads manager in Firefox (here is the full list). User is not able to change it (OK, as for now he's able but the add-on that lets him do it will not work on Firefox 57+). Could you provide a shortcut that does not conflict with default (and unchangeable) browser shortcuts?
The page you linked does not show Ctrl + Shift + Y as being a shortcut for anything in Firefox...
It does.

Sorry, I haven't found a way to link to the certain shortcut on this page.
Weird. Here is what mine says:

That depends on useragent because default shortcuts on different systems differ.

Same here, may I suggest to also switch to a shorter shortcut (I personally use it a lot).
My suggestion would be Alt+W
Also affected, Firefox on Linux.
A similar sequence to lastpass would be great: Alt+PgDn / Alt+pgUp for next/prev matching password, which is handy for sites with multiple password matches.
That's mostly browser fault but not extension one. Firefox does not allow changing the extension keyboard shortcuts (though the semantics of the manifest.json entry name seems to imply that the default key could be changed). So, I think the best idea is to change the shortcut to something else (like @xdevla's suggestion) only for GNU/Linux leaving it the same for all other platforms (and therefore majority of users)
I have adjusted this for next release. The follow commands with be available as the defaults:
Ctrl+Shift+UCtrl+Shift+Y (unchanged)Ctrl+Shift+LCtrl+Shift+Period (not available on Firefox)kspearrin just tell me what you are smoking there... Ctrl+Shift+U is shortcut to enter unicode, try to use your crap while on a textbox firefox+ubuntu
@Unixware it's not a Firefox shortcut. I'm not the developer but I think if there's conflicting shortcut in your DE (there are too many of them for GNU/Linux to adapt to all of them), it's up to you to change the keyboard shortcut for the extension. Recent versions of Firefox allow this, so this should not be a problem.
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kspearrin just tell me what you are smoking there... Ctrl+Shift+U is shortcut to enter unicode, try to use your crap while on a textbox firefox+ubuntu